Tehranchi et al. evaluated hematopoietic stem and progenitor cell (HSPC) populations in del (5q) MDS patients treated with lenalidomide in remission and relapse (Tehranchi et al., 2010) and identified rare and phenotypically distinct del (5q) MDS-SCs (CD34+CD38−/low CD90+) that are resistant to lenalidomide treatment (Tehranchi et al., 2010). This evidence concerns the gene THY1 and myelodysplastic syndrome.